Wastewater treatment process and apparatus for high flow fluoride removal

ABSTRACT

A process and system for removing heavy metals, fluoride, silica and other contaminants from large volumes of wastewater is disclosed. In the process, a wastewater stream containing the contaminant is treated with a chemical coagulant to create a particle having a diameter greater than 5 microns. Treated wastewater is passed through a microfiltration membrane which physically separates the metal contaminant particle from the wastewater. Commercially available microfiltration membranes having a pore size from 0.5 micron to 5 microns may be used. The treated wastewater flow rate through the microfiltration membranes can range from 700 gallons per square foot of membrane per day (&#34;GFD&#34;) to 1500 GFD. Solids are removed from the membrane surface by periodically backflushing the microfiltration membranes and draining the filtration vessel within which the membranes are located. The dislodged solid material within the filtration vessel is flushed into a holding tank for further processing of the solids.

This application is a divisional of application Ser. No. 08/756,681,filed Nov. 26, 1996.

FIELD OF THE INVENTION

The present invention relates to the treatment and purification ofwastewater at high flow rates. More particularly, the present inventionrelates to process and apparatus for removing heavy metal and certainnon-metal contaminants from large quantities of wastewater.

BACKGROUND OF INVENTION

Many manufacturing operations generate extremely large quantities ofwater containing heavy metals or other contaminants. For instance,mining drawdown wells which are used to dewater deep mining operationsare known to generate up to 75,000 gallons per minute (gpm) of water.Often this water contains heavy metals or other impurities which must beremoved from the water before it can be safely discharged into theenvironment.

Current techniques for treating drawdown wastewater include largesettling ponds, clarifiers, and sand filter systems utilizing iron oraluminum chemistry with large quantities of polymer additives. Suchsystems are able to demonstrate 90% compliance to discharge regulations.For example, arsenic cannot be safely discharged into the environmentunless its concentration is less than 50 ppb ("parts per billion"). Ifinfluent arsenic levels are greater than 300 ppb, clarifier and sandfilter systems are not able to consistently provide discharge levelsless than 50 ppb. To achieve this level of arsenic reduction, chemicalcoagulants are required to form heavy and large particles, typicallygreater than 200 microns in size. However, such systems are subject tobiological fouling, sand settling, and upsets. Upsets result in out ofcompliance water. In addition, system maintenance is extensive, withvery large land areas required for the system installation.

Filters have been considered to remove metal contaminants fromwastewater. For example, traditional microfiltration membranes have apore size of about 0.5 microns with a flux rate of 100-200 GFD ("gallonsper square foot of membrane per day"). At this flux rate, it would benecessary to have a membrane of at least 180,000 square feet to process25,000 gpm of wastewater. If the wastewater flow rate is 75,000 gpm,then the membrane size would need to be at least 540,000 square feet.Such membrane sizes are prohibitively large and expensive.

It would be a significant advancement in the art to provide a processand system for removing metals and other contaminants from largequantities of wastewater.

It would be a major advancement in the art to provide a process andsystem for removing metals and other contaminants from large quantitiesof wastewater which do not require large land areas.

It would also be an important advancement in the art to provide aprocess and system for removing metals and other contaminants from largequantities of wastewater which consistently complies with environmentaldischarge requirements.

Such processes and systems are disclosed and claimed herein.

SUMMARY OF THE INVENTION

The present invention is directed to a process for removing metal andcertain non-metal contaminants from large volumes of wastewater. In theprocess, a wastewater stream containing a contaminant is treated with achemical coagulant. Typical metal contaminants found in mining and otherindustrial wastewater streams include silver (Ag), arsenic (As), gold(Au), barium (Ba), cadmium (Cd), chromium (Cr), copper (Cu), mercury(Hg), nickel (Ni), lead (Pb), zinc (Zn), fluoride (F⁻), and silica(SiO₂). The present invention can readily be adapted for removing othermetals and contaminants found in wastewater by using suitable coagulantchemistry. The coagulant reacts with the contaminant to form aparticulate having a size greater than about 5μ.

Known and novel chemical coagulants are available to achieve the desiredparticulate formation. For instance, ferric sulfate, ferrous sulfate,aluminum sulfate, sodium aluminate, and aluminum and iron polymers arewell known inorganic coagulants. Organic and polymeric coagulants canalso be used, such as polyacrylamides (cationic, nonionic, and anionic),epi-dma's (epi-dimethylamines), DADMAC's (polydiallydimethylammoniumchlorides), copolymers of acrylamide and DADMAC, natural guar, etc. Somecoagulants, such as boro-hydrides, are selective for certain metals. Thestoichiometric ratio of coagulant to metal or non-metal contaminant ispreferably optimized to result in acceptable contaminant removal atminimum coagulant cost. The required coagulant concentration will dependon several factors, including metal contaminant influent concentration,wastewater flow rate, metal contaminant effluent compliance requirement,coagulant/contaminant reaction kinetics, etc. For metal contaminants,the ratio of coagulant to metal contaminant is typically in the rangefrom 3.1 to 16.1. Arsenic, for example, requires a 6:1 to 10:1 (ferriccoagulant: arsenic) ratio, lead requires a 3:1 to 8:1 coagulant:metalratio, zinc uses about 4:1 coagulantugmetal ratio, while coppertypically requires a coagulant:metal ratio in the range from 3:1 to 8:1.Fluoride and silica typically have a ratio of coagulant to contaminantin the range from 2:1 to 30:1, depending on the system.

Treated wastewater is passed through a microfiltration membrane whichphysically separates the metal contaminant from the wastewater. Suitablemicrofiltration membranes are commercially available from manufacturerssuch as W. L. Gore, Koch, and National Filter Media (Salt Lake City,Utah). For instance, one GOR-TEX® membrane used in the present inventionis made of polypropylene felt with a sprayed coating of teflon. Theteflon coating is intended to promote water passage through themembrane. Such microfiltration membrane material has been found to beuseful for many wastewater treatment systems. However, when used in asystem for removing fluoride or silica, it has been observed that thecoagulated particles adhere to the exterior and interior surface andplug the membrane. Backflushing was not effective in such cases.

The microfiltration membranes are used in a tubular "sock" configurationto maximize surface area. The membrane sock is placed over a slottedtube to prevent the sock from collapsing during use. A net material isplaced between the membrane sock and the slotted tube to facilitate flowbetween the membrane and the slots in the tube. In order to achieve theextremely high volume flow rates, a large number of membrane modules,each containing a number of individual filter socks, are used.

The microfiltration membranes preferably have a pore size in the rangefrom 0.5 micron to 5 micron, and preferably from 0.5 micron to 1.0micron. By controlling the ratio of coagulant to metal contaminant,99.99% of the precipitated contaminant particles can be greater than 5microns. This allows the use of larger pore size microfiltrationmembranes. It has been found that the treated wastewater flow ratethrough 0.5 to 1 micron microfiltration membranes can be in the rangefrom 700 gallons per square foot of membrane per day ("GFD") to 1500GFD.

Solids are preferably removed from the membrane surface by periodicallybackflushing the microfiltration membranes and draining the filtrationvessel within which the membranes are located. The periodic, shortduration back flush removes any buildup of contaminants from the wallsof the microfiltration membrane socks. The dislodged solid materialwithin the filtration vessel is flushed into a holding tank for furtherprocessing of the solids.

The wastewater treatment system disclosed herein is designed to providecompliance with the contaminant metal discharge effluent limits.Wastewater pretreatment chemistry creates insoluble metal and non-metalcontaminant particulates which are efficiently removed by themicrofiltration membranes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of one wastewater pretreatmentsystem.

FIG. 2 is a schematic representation of one wastewater microfiltrationapparatus for high flow impurity removal.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a process for removing metal andcertain non-metal contaminants from large volumes of wastewater. Inoperation, the wastewater is collected and pretreated with one or morechemical coagulants such that the contaminant reacts with thecoagulant(s) to form particulates having a size greater than about 5μ.The chemical coagulants are preferably mixed with the wastewater usingreaction vessels or static in-line mixers, although other mixing methodscan be used.

The treated wastewater is then passed through a microfiltration membranehaving a pore size in the range from 0.5μ to 5μ to remove thecontaminant particulates. In such a system, wastewater flow rates in therange from 700 gallons per square foot of membrane per day ("GFD") to1500 GFD are possible. The microfiltration membrane is periodicallybackflushed to remove solids from the membrane surface. The rejectedsolids are gravity collected in the filter vessel bottom and time cycledischarged to a settling tank for further sludge processing.

The microfiltration membranes are preferably provided in a cassettearranged module. The microfiltration membranes provide a positiveparticle separation in a high recovery dead head filtration array. Thedead head filtration operates effectively at low pressures (4 psi to 15psi, preferably 5 psi to 10 psi) and high flow rates, allowing 100%discharge of the supplied water with no transfer pumps needed. Solidswhich settle on the wall of the membrane during filtration areperiodically backflushed away (and gravity settled) from the membranesurface to ensure a continuously clean filtration area. The individualcassette module design allows for easy replacement of the membranemodules.

Currently preferred filter socks useful with the present inventioncontain a teflon coating on a polypropylene or polyethylene felt backingmaterial. Such socks are available from W. L. Gore. Another presentlypreferred filter sock manufactured by National Filter Media, Salt LakeCity, Utah, consists of a polypropylene woven membrane bonded to apolypropylene or polyethylene felt backing. Membrane "failure" is dueprimarily to flux rate loss, not mechanical failure. Many operationsdeem it more cost-effective to replace the membrane socks instead ofcleaning contaminants from the membrane.

The membrane life is important to the continuous operation andoperational cost of the filtration system. The membranes manufactured byW. L. Gore and National Filter Media, Salt Lake City, Utah have a 6 to24 month life with no catastrophic failures in industrial conditions ata temperature of 160° F. and a pH greater than 13. Anticipated operatingconditions for the present invention are ambient temperature and pHbetween 7 and 11. It is expected that membranes used according to thepresent invention will have a life equal to or greater than 18 months.The filtration system operates at a low pressure, preferably between 4and 15 psi. Greater pressures are possible; however, the higher thepressure, the quicker the membrane loss of flux rate. The operatingpressure is preferably below 25 psi.

The following examples are offered to further illustrate the presentinvention. These examples are intended to be purely exemplary and shouldnot be viewed as a limitation on any claimed embodiment.

EXAMPLE 1

Using a 50 gallon per minute (gpm) pilot scale system, actual minedraw-down wastewater containing arsenic contaminant was processedaccording to the present invention. Ferric sulfate (at a ratio of 8:1Fe:As) was used as the coagulant. DADMAC ((poly) diallyldimethylammoniumchloride) and a copolymer of acrylamide and DADMAC were used at aconcentration of 1 ppm (parts per million). The DADMAC was used as a 20%liquid and the DADMAC acrylamide copolymer was used as a 10% liquid. Themembrane was obtained from W. L. Gore having a teflon coating and anominal pore size range of 0.5μ. The flux rate ranged from 430 to 600GFD at an operating pressure less than 10 psi. The results are reportedbelow in Table

                  TABLE 1    ______________________________________    All Values are in Parts Per Billion (ppb)           Arsenic Influent                         Arsenic Effluent    Time Period             Mean    High    Low   Mean  High  Low    ______________________________________    A        331     429     247   13.3  82    0    B        270     375     165   5.3   15    0    C        279     369     231   7.0   24    0    D        278     278     278   2.7    7    0    E        244     268     197   4.9   14    0    ______________________________________

EXAMPLE 2

A 15 gpm pilot scale system was used to process wastewater containingfluoride and a combined flow of fluoride and silica. A 38% sodiumaluminate solution at a ratio of 0.23:1 Al:F and 50% aluminumchlorohydrate at a dose of 35 ppm to aid in the removal of the fluoride,total dissolved solids (TDS), total suspended solids (TSS), and some ofthe other present salt forms. The precipitate was flocculated with amedium charge (25±5 mole percent), medium molecular weight anionicpolyacrylamide polymer for ease of filtering or settling. This yieldedvery low to non-detectable effluent values of fluoride and Silt DensityIndices (SDI) below 3.0. The filtration membrane was a 0.5μpolypropylene bonded membrane obtained from National Filter Media. Themembrane flux rate was measured at 650 to 800 GFD at a vessel operatingpressure less than 9 psi. The results are reported below in parts permillion.

    ______________________________________    Time Period  Influent F  Effluent F    ______________________________________    A            130.0       1.86    B            191.5       21.7    C            142.2       2.13    D            120.0       0.72    E            156.5       1.41    F            125.7       0.79    G            60.93       0.97    H            206.25      0.95    I            133.3       0.39    J            112.9       0.85    K            78.2        3.96    L            133.5       3.96    Average      132.6       3.8    Min          60.93       0.39    Max          206.25      21.7    ______________________________________    Time Period  Influent F + SiO.sub.2                             Effluent F + SiO.sub.2    ______________________________________    A            264.0       0.24    B            172.0       0.26    C            140.0       0.31    D            153.0       0.39    E            98.0        0.36    F            89.0        0.29    Average      152.7       0.31    Min          89.0        0.24    Max          264.0       0.39    ______________________________________

EXAMPLE 3

A 15 gpm pilot scale system was used to process wastewater containingsilica. The silica was present in dissolved and colloidal silica forumin the waste stream. A 38% sodium aluminate solution at a ratio of0.45:1 Al:Si, 46% aluminum sulfate at constant dose of 45 ppm, 50%aluminum chlorohydrate at a dose of 25 ppm, and a 20%epichlorohydrin/dimethylamine (a high charged, low molecular weightcationic epi-DMA product) at a dosage of 0.25-1.0 ppm to aid in theremoval of the silica, TDS and TSS. This formed a well defined particlefor filtering or settling. This yielded very low to non-detectableeffluent values of the silica and Silt Density Indices (SDI) below 3.0.The filtration membrane was a 0.5 micron polypropylene felt with a PTFE(polytetrafluroethylene coating obtained from W. L. Gore. The membraneflux rate ranged from 500 GFD to 900 GFD at a vessel operating pressureless than 9 psi. The results are reported below in parts per million.

    ______________________________________    Time Period   Influent SiO.sub.2                            Effluent SiO.sub.2    ______________________________________    A             140       0.443    B             160       0.33    C             125       0.37    D             153       0.39    E             177       0.36    F             165       0.29    Average       153       0.364    Min           125       0.29    Max           177       0.443    ______________________________________

EXAMPLE 4

A 15 gallon per minute (gpm) pilot scale system was used to processwastewater containing copper and lead in a combined waste flow. Thecopper and lead removal system employed the use of a blend of sodiumthiocarbonate and sodium aluminate which was fed at a ratio of 3.2:1(thiocarbonate to combined metal concentration of copper and lead asmeasured by atomic absorption). The precipitate was flocculated with amedium charge, medium molecular weight polyacrylamide polymer for easeof filtering or settling. This yielded a very low to non-detectableeffluent values of copper and lead in the effluent. The membrane was a1.0 micron polypropylene needled monoelement obtained from NationalFilter Media. The membrane flux rate was estimated to be 1000 GFD atvessel pressures from 4.5 to 6.0 psi. The results are reported below inparts per million:

    ______________________________________             Lead             Copper    Time Period               Influent                       Effluent   Influent                                        Effluent    ______________________________________    A          3.2     0.11       28.0  N.D.    B          2.85    0.14       32.98 0.032    C          3.66    0.109      21.31 0.045    D          2.45    0.15       23.0  0.023    E          3.0     0.10       28.0  N.D.    F          2.4     0.09       35.0  N.D.    G          3.8     N.D.       35.11 0.07    H          2.76    0.10       33.0  0.055    I          4.12    N.D.       27.27 0.11    J          2.65    0.12       24.6  N.D.    Average    3.09    0.09       28.82 0.0335    Min        2.4     N.D.       21.31 N.D.    Max        4.12    0.15       35.11 0.11    ______________________________________

Reference is made to FIG. 1 which illustrates one possible wastewaterpretreatment system 10 within the scope of the present invention. Theillustrated wastewater pretreatment system 10 includes a plurality ofpretreatment reactor vessels 12, 14, and 16 which enable the wastewaterfeed stream 18 to chemically react with one or more chemical coagulants.Chemical coagulants which react with contaminants in the wastewater feedstream 18 are introduced into the pretreatment reactor vessels viachemical coagulant feed streams 20, 22, and 24. The pH within thepretreatment reactor vessels is preferably monitored with a pH sensor26. Acid or base can be added to the pretreatment reactor vessels, ifnecessary, to adjust the pH via acid/base feed stream 28.

The number of pretreatment reactor vessels can vary depending on thenumber of chemical coagulants being used and the reaction chemistry usedto form the waste particulates. The size of the reactor vessels can bevaried to provide different reaction times.

After flowing through the necessary pretreatment reactor vessels, thewastewater feed stream flows into a feed tank 30 for holding thepretreated wastewater. Additional chemical coagulants can be addeddirectly to the feed tank 30, if necessary, via a chemical coagulantfeed stream 31. As shown in FIG. 2, the pretreated wastewater isdirected to one or more filtration vessels 32, 34, and 36 via filtrationvessel feed stream 38. The size of feed stream 38 will depend on thedesigned flow rate of the filtration vessel. For example, in a systemhaving 5 filtration vessels, each handling 2500 gpm, a 24 inch feed lineto the system is suitable. Each filtration vessel 32, 34, and 36 is astand alone filtration device. The number and size of each filtrationvessel can vary depending on the system capacity requirements. Thefiltrate is removed from each filtration vessel via a filtrate stream40.

Each filtration vessel preferably provides a mounting platform for from9 to 24 filter cassette modules. One currently preferred filter cassettemodule contains 16 individual sock filters configured with 0.5 micronfiltration membranes. The rated flow rate is 0.9 gpm per square foot ofmembrane area. Each full cassette module has 64 square feet of membranearea and is rated at 58 gpm with a differential pressure less than 15psi. A lifting mechanism is preferably included to allow removal andreplacement of the membrane cassette modules.

The filtration membranes are periodically backflushed with filtrate toremove solids from the membrane surface. During the backflush procedure,the filtration vessel is taken off line and wastewater is drained fromthe filtration vessel via a backflush exit stream 42 to a backflush tank44. The backflush tank 44 provides temporary storage before thebackflushed wastewater is conveyed to the feed tank 30 via backflushreturn stream 46. It is estimated that 400-500 gallons of water will beused during a typical back flush cycle for a 2500 gpm filtration vessel.A vacuum breaker 48 is preferably provided to allow equalization ofpressure within the respective filtration vessel 32, 34, or 36 duringthe backflush procedure. A vent/relief stream 49 is provided to allowventing or release of excess or over-pressurized wastewater.

The filtrate side of the filtration vessel 32, 34, 36 is open to theatmospheric pressure. The filtrate is collected in the top of thefiltration vessel and allowed to drain into the filtrate stream 40. Thisvolume of water provides the positive head which, when coupled with thenegative head of draining the pressure side of the vessel via backflushexit stream 42, produces enough positive pressure gradient to backflushthe filtration membrane.

After sufficient sludge settles within the bottom of the filtrationvessel 32, 34, 36, the sludge is removed via a sludge discharge stream50. While the sludge is removed, the filtration membranes are preferablyrinsed with water from a water rinse stream 52. The collected sludge isremoved from the system for further processing or storage.

Periodically, the membranes will require soaking to remove trace amountsof organics. Cleaning preferably occurs as needed or as part of aregular maintenance program. The vessel drain opens to remove allcontaminant via the sludge discharge stream 50. The cleaning solution isintroduced into each filtration vessel through cleaning supply stream54. Typical cleaning solutions include acids, bases and surfactants. Insome cases the filtration vessel can be returned to operation withoutdraining and rinsing the filtration membranes. If membrane rinsing isnecessary, the contents of the filtration vessel 32, 34, 36 are removedvia cleaning discharge stream 56 for further processing.

As shown in FIG. 2, multiple filtration vessels are preferably used, inparallel, to provide for the required flow rate. However, the filtrationvessels can be operated in series to provide primary filtration andsecondary filtration. Because filtration vessels are taken off lineduring the backflushing, additional filtration vessels and capacity arepreferably used to ensure that the require discharge flow is maintained.An additional filtration vessel may be supplied to provide for off-linemaintenance while the remainder of the system meets the flow raterequirements.

The wastewater treatment system preferably includes access to thevarious process streams to allow for sampling and analysis. The valves,pumps, and sensors customarily used in the art to safely control thedescribed fluid flow to and from the filtration vessels are preferablyprovided. Such valves, pumps, and sensors also allow for automation ofthe process.

From the foregoing, it will be appreciated that the present inventionprovides a process for removing contaminants from wastewater utilizing apositive physical barrier to precipitated particles. The positiveseparation barrier permits discharge having lower concentration limitsthan conventional clarifier/sand filter systems.

The apparatus for removing contaminants from wastewater occupies lessspace than conventional clarifier/sand filter systems. The apparatus iseasily expandable.

The chemical pretreatment achieves particle formation based on size, notweight. As a result, chemical pretreatment costs are lower than thosetypically required for a clarifier/sand filter.

The present invention may be embodied in other specific forms withoutdeparting from its essential characteristics. The described embodimentsare to be considered in all respects only as illustrative and notrestrictive. The scope of the invention is, therefore, indicated by theappended claims rather than by the foregoing description.

The claimed invention is:
 1. A process for removing fluoride from largevolumes of wastewater comprising the steps of:(a) treating a wastewaterstream containing fluoride with a coagulant, wherein the coagulantreacts with the fluoride to form a particulate having a size greaterthan about 5μ; (b) passing the treated wastewater through amicrofiltration membrane having a pore size in the range from 0.5μ to5μ, wherein the treated wastewater flow rate is in the range from 700gallons per square foot of membrane per day ("GFD") to 1500 GFD, suchthat the fluoride is removed from water passing through themicrofiltration membrane; and (c) periodically backflushing themicrofiltration membrane to remove solids from the membrane surface. 2.A process according to claim 1, wherein the mole ratio of coagulant tofluoride is in the range from 2:1 to 3:1.
 3. A process according toclaim 1, wherein the coagulant is aluminum chloride aluminumchlorohydrate, and sodium aluminate.
 4. A process according to claim 1,wherein the coagulant is a sodium aluminate solution at a ratio of from0.2:1 to 5:1 Al:F and aluminum chlorohydrate at a dose of from 30 to 40ppm.
 5. A process according to claim 4, further comprising the step ofadding from 20 to 30 mole percent, relative to the fluoride content,medium molecular weight anionic polyacrylamide polymer.
 6. A processaccording to claim 1, wherein the microfiltration membrane comprisespolypropylene felt with a coating of polytetrafluoroethylene (PTFE). 7.A process according to claim 1, wherein the microfiltration membranecomprises polypropylene membrane bonded to a polypropylene orpolyethylene felt backing.
 8. A process according to claim 1, whereinthe treated wastewater is passed through the microfiltration membrane ata pressure less than 25 psi.
 9. A process according to claim 1, whereinthe treated wastewater is passed through the microfiltration membrane ata pressure in the range from about 4 psi to 15 psi.
 10. A processaccording to claim 1, wherein the treated wastewater is passed throughthe microfiltration membrane at a pressure in the range from about 5 psito 10 psi.
 11. A process according to claim 1, wherein the coagulant isa polyacrylamide.
 12. A process according to claim 1, wherein thecoagulant is an epichlorohydrin/dimethylamine (epi-dma) polymer.
 13. Aprocess according to claim 1, wherein the coagulant is a DADMAC(polydiallydimethylammonium chloride) polymer.
 14. A process accordingto claim 1, wherein the coagulant is a copolymer of an acrylamide andDADMAC (polydiallydimethylammonium chloride).
 15. A process according toclaim 1, wherein the coagulant is a natural guar.
 16. A process forremoving fluoride from large volumes of wastewater comprising the stepsof:(a) treating a wastewater stream containing fluoride with an organicpolymer coagulant, wherein the coagulant reacts with the fluoride toform a particulate having a size greater than about 5μ; (b) passing thetreated wastewater through a microfiltration membrane having a pore sizein the range from 0.5μ to 5μ, wherein the treated wastewater flow rateis in at least 650 gallons per square foot of membrane per day ("GFD"),such that the fluoride is removed from water passing through themicrofiltration membrane; and (c) periodically backflushing themicrofiltration membrane to remove solids from the membrane surface. 17.A process according to claim 16, wherein the coagulant is apolyacrylamide.
 18. A process according to claim 16, wherein the treatedwastewater is passed through the microfiltration membrane at a pressureless than 25 psi.
 19. A process according to claim 16, wherein thetreated wastewater is passed through the microfiltration membrane at apressure in the range from about 4 psi to 15 psi.